This invention relates to a method for manufacturing a magnetic yoke assembly for a magnet-type electric motor including a hollow cylindrical magnetic yoke and a plurality of permanent magnets attached to the inner peripheral surface of the yoke. Such motor can be used as a starter motor for an internal combustion engine, for example.
FIGS. 1 and 2 illustrate one example of a typical magnet-type starter motor for an internal combustion engine including a magnetic yoke assembly which can be manufactured by the manufacturing method of the present invention. The magnet-type motor 10 comprises a hollow cylindrical magnetic yoke assembly 11 which also serves as a housing for the motor 10. As seen from FIG. 2, which is a cross sectional view taken along the line II--II of FIG. 1, the magnetic yoke assembly 11 comprises a magnetic yoke 12 which is provided on its inner peripheral surface 14 with a plurality of field permanent magnets 16 attached at equal intervals thereto. In the illustrated example, there are six field magnets 16. The magnetic yoke 12 which is closed at one end by a cup-shaped member 18 also supports therein an armature 20 having a commutator assembly 22 and brush assemblies 24. The motor 10 is connected to an overruning clutch 26 which in turn is connected to a pinion 28 which is axially slidable between an engaged position in which the pinion 28 engages a ring gear (not shown) of the engine and a disengaged position in which the pinion 28 disengages from the ring gear (not shown). In order to axially slide the pinion 28 between the engaged and disengaged positions, the electromagnetic switch 30 which rocks a drive lever 32 about a fulcrum 34 to slidably move the pinion 28 is provided.
The magnetic yoke 12 is manufactured from a hollows cylindrical tube 40 (FIG. 3) of a magnetic material. During manufacture of the cylindrical magnetic yoke 12, it is necessary to expand the diameter of the magnetic yoke 12 in order to precisely attach the field permanent magnets 16 on the inner peripheral surface of the magnetic yoke 12. This expanding is achieved as illustrated in FIG. 3 by first applying a sliding sheet 42 on the inner peripheral surface of the cylindrical tube 40, and then inserting a tube expander 44 into the tube 40. The tube expander 44 has a plurality of expander elements 46 capable of being moved radially outwards by a hydraulic pressure for example to expand the radius of the tube 40. Then, each of the expander elements 46 are moved radially outwards moved by the hydraulic pressure to plastically deform the tube 40, as shown in FIG. 2, in accordance with the configuration of the convexed cylindrical surfaces of the expander elements 46 to form a plurality of cylindrical concaved surfaces 48 separated by axially extending ridges 50 in the inner peripheral surface of the tube 40. Therefore, the number of the concaved surfaces 48 thus formed is equal to the number of the expander elements 46 used.
According to the conventional method, the tube expander 44 used is a commercially available expander which is provided with eight expander elements 45 as shown in FIG. 3, and no attention is paid to the relationship between the number of the expander elements 46 (typically eight) and the number of the permanent magnets 16 (typically six) to be attached to the inner surface of the magnetic yoke 12. This is quite natural because the purpose of using the expander is to expand the inner diameter of the tube to a precise desired dimension.
After the tube 40 has been expanded, the permanent magnets 16 are secured to the expanded inner surface of the tube 40 as shown in FIG. 2. However, since the number of the cylindrical recesses 48 formed by the expander elements 46 is eight and the number of the permanent magnets 16 to be attached is six, the outer cylindrical surface 54 of the permanent magnets 16 do not properly fit in a surface-to-surface relationship to the inner surface of the tube 40 and small gaps or clearances 52 are formed between the recesses 48 of the tube 40 and the other surfaces 54 of the permanent magnets 16 as illustrated in a somewhat exaggerated manner in FIG. 2. It is also seen from FIG. 3 that the permanent magnets 16 are in contact with the tube 40 only at the ridges 50 between the recesses 48, resulting in a poor magnetic coupling between them. Therefore, according to the conventional manufacturing process, the permanent magnets 16 must be pressed against the inner surface of the tube 40 with a large force, which sometimes causes damages such as cracks to the permanent magnets 16.